CREATE OR REPLACE FUNCTION extract_value(
   VARCHAR,
   XML
) RETURNS TEXT AS
$$
   SELECT CASE WHEN $1 ~ '@[[:alnum:]_]+$'
   THEN (xpath($1, $2))[1]
   WHEN $1 ~* '/text()$'
   THEN (xpath($1, $2))[1]
   WHEN $1 LIKE '%/'
   THEN (xpath($1 || 'text()', $2))[1]
   ELSE (xpath($1 || '/text()', $2))[1]
   END::text;

$$ LANGUAGE 'sql' IMMUTABLE;